This invention relates to the medical arts. In particular, it relates to an apparatus for measuring fluids such as end-tidal carbon dioxide and a method for using the same.
In numerous clinical settings, such as under anesthesia or during artificial respiration, it becomes desirable to monitor the carbon dioxide concentration in the arterial blood gas of a patient. Invasive procedures have been designed for accomplishing this, which include periodic blood sampling of arterial blood gas and the use of an in-dwelling catheter capable of directly monitoring the carbon dioxide concentration. The fact that such techniques are invasive subjects them to all the problems usually associated with such procedures, including the increased risk of infection, thrombosis, etc.
It is known that the carbon dioxide concentration of the last gas expired from the lung (alveolar or end-tidal gas) in normal breathing is related to the carbon dioxide concentration of arterial blood gas. In a single exhalation cycle, the first portion of such mixture exhaled by the patient consists principally of ambient air in passageways between the point of exhalation and the main airways of the lung. The first portion merges into the second portion of the exhaled mixture which consists of the residual ambient air and end-tidal gas (i.e., the gas contained in the cells of the lung). The last portion of the exhaled mixture consists principally of the end-tidal gas. This last portion of the exhaled mixture is of primary interest for diagnostic and analytical purposes.
Capnography is the monitoring of end-tidal carbon dioxide (CO.sub.2) concentration. A capnometer measures the amount of carbon dioxide gas exhaled by a patient. Typically, a sampling tube is used to convey the sampled flow of gas from the air tube to the gas measuring means. The sampling tube can be connected to a sampling port located outside or just inside the external narier or, where a patient is fitted with an endotracheal tube, the sampling tube can be integral with the endotracheal tube. However, medical instruments relying upon such tubes are not as accurate or as versatile as might be desired.
It is an object of the present invention to provide improved apparatus and methods for use in the measurement of a fluid in a patient.